Changing Apple ID on MacBook Air?

Me and my sister share the same Apple ID; her iPod and my laptop. This is causing our messages and things in general, to mix up. For example, her iMessage messages will transfer onto my laptop. I created a new Apple ID, but I don't know how to change it on my laptop. We both want to get rid of the old Apple ID and have our own individual ones. Would I have to restart my laptop so I can add it in?

Thanks